A voltage controlled oscillator or VCO is an electronic oscillator designed to be controlled in oscillation frequency by a voltage input, i.e. the frequency of oscillation can be varied by the applied tuning voltage. However, an unwanted drift of oscillation frequency can be observed due to a temperature dependent variation of physical parameters of the device. For example, the performance of a VCO being integrated on Silicon (Si) substrate is affected by the temperature of the substrate. A VCO designed for Giga-Hertz operation has all components which determine the oscillation frequency integrated on the substrate. The electrical parameters of the components and the mechanical chip dimensions vary with temperature. When the VCO chip temperature is increased—either by operating in a hot environment or by self-heating caused by dissipated power of the circuit—the oscillation frequency changes. If the frequency of operation is comparably low (about 2 GHz, for example), a VCO resonator, i.e. the VCO oscillator core, may contain a lumped inductor, which is comprised of a coil made by the metal layers of the semiconductor process. At higher frequency (>15 GHz, for example), a VCO resonator is realized with transmission lines (TL) located on chip. The length of the TL influences the frequency of operation. When the VCO chip is heated up, the mechanical dimensions of the TL increase according to the thermal expansion coefficient of the substrate. As a consequence, the frequency of operation of the VCO is shifted to a lower value.
This effect can be quite substantial, if a VCO being implemented with integrated TLs is operated at millimeter wave frequency. A measured example is shown in FIG. 1: A state-of-the-art VCO circuit with microstrip TLs on Si-substrate oscillating at 77.0 GHz at 25° C. experiences a drift of approximately 2.0 GHz towards lower frequency when being heated up to 125° C. Besides a variation of the mechanical device dimensions, other physical parameters of a VCO might be subject to a change caused by a change of the ambient temperature. For example, if a VCO core is implemented using bipolar transistor technology, the capacitance of the collector base diodes changes with temperature, causing a frequency drift of the VCO. Resistors used in the VCO core change their resistance values depending on temperature, therefore influencing a drift of the VCO frequency. Furthermore, the loop gain of a VCO may be subject to change, since the transit time of a transistor, for example a heterojunction bipolar transistor (HBT) that can handle signals of very high frequencies, decreases with increasing temperature, causing a reduced frequency of the VCO.
The temperature drift of integrated VCOs is a known issue. For example, the document EP 0 580 209 A1 describes a low-gain, range programmable, temperature compensated voltage controlled oscillator, wherein a VCO must be contained in a phase locked loop. The oscillator signal is made substantially temperature-independent by requiring a temperature-independent constant current source providing a fixed current through a variable resistance and a temperature dependent current which sinks and sources additional current to the oscillator as temperature rises and falls. The document U.S. Pat. No. 6,043,720 describes an oscillator architecture and a temperature compensation circuit with a frequency drift compensation based on varying various different biasing parameters. No integrated varactors are used. The document U.S. Pat. No. 6,052,036 describes a crystal oscillator with automatic gain control and on-chip tuning. A fixed frequency oscillator employing a crystal and using a PTAT circuit (PTAT: Proportional to Absolute Temperature) for temperature compensation is provided. However, PTAT circuits allow for compensation of a linear variation of a voltage over temperature. The influence of second order non-linear terms, which play a significant role when a desired temperature coefficient of a compensation voltage is supposed to be very low, is not compensated sufficiently. The document U.S. Pat. No. 5,151,667 shows a temperature compensated non-symmetrical oscillation circuit without employing varactors for a compensation. The document U.S. Pat. No. 4,270,102 provides another non-symmetrical oscillation circuit with a temperature compensation. The document U.S. Pat. No. 4,978,930 describes a VCO temperature compensation circuit based on a PTAT current source. The VCO is provided with a temperature dependent offset for the tuning voltage. However, the attachment of the temperature compensation circuit to the VCO resonator degrades the phase noise of the oscillator. The document U.S. Pat. No. 4,833,426 describes a temperature compensated piezoelectric oscillator requiring thermistors. Hence, components not suitable for the implementation of an integrated VCO operating at high frequencies are required.